A coating system becoming increasingly popular, particularly in the automotive industry, is one known as "color plus clear". In this system the substrate is coated with one or more applications of a pigmented basecoating composition to form a basecoat which thereafter is coated with one or more applications of an essentially clear topcoating composition to form a topcoat.
However, there are several disadvantages with a number of known color plus clear coating systems. After conventional basecoating compositions are applied to the substrate, a rather long period of time, on the order of about 30 minutes or more, may be required between the application of the conventional basecoating composition and the conventional topcoating composition. Such a period is needed to prevent adverse attack by components of the conventional topcoating composition, particularly solvents, on the basecoating composition at the interface of the two, a phenomenon often referred to as strike-in. Strike-in adversely affects the final appearance properties of the coated product. Strike-in is an especially serious problem when metallic-flake pigments are employed in the basecoating composition. Strike-in, among other things, can destroy the desired metallic-flake orientation in the basecoat. Moreover, strike-in can adversely affect the overall clarity and distinctness of image in the resulting cured composite film.
Often, known color plus clear systems based on thermosetting resins require elevated temperatures typically of at least 120 degrees Celsius (degrees C.) for curing. It is desirable to provide color plus clear coating methods which utilize relatively low temperatures, for example, below about 82 degrees C., and preferably ambient temperatures. A number of previous attempts to develop such coating systems resulted in systems which had the disadvantages of being too time consuming and/or energy intensive or resulted in cured films which were deficient in various combinations of physical properties.
In accordance with the present invention, a color plus clear coating system has been developed which can provide an acceptable rate of cure at low or even ambient temperatures and results in coated products in which the films exhibit a surprisingly excellent combination of appearance and physical properties, particularly a surprisingly excellent distinctness of image (DOI). A film having a high degree of distinctness of image when viewed from a direction close to the normal to the surface and under, for example, a light fixture such as a fluorescent light fixture having a cross-hatch grid in front of the bulb, exhibits a reflected image of the lighted fixture in the film which appears clear and sharply distinct and which appears to originate deep in the film.